Hello

1995 850 Turbo 73,000 miles. I was driving home from work the other day, and as I neared my apartment I noticed steam coming from the right side of the hood. The temperature gauge on the dash indicated normal temperature. I opened up the hood, to find coolant steaming/spilling out of the top of the coolant container. Let it cool down overnight. Coolant level shown as being below minimum. Topped off with water. Drove about five miles. Same result. Still temperature gauge showing normal temp. Came back 3 hours later, topped off with water, drove the five miles back home. Checked under the hood; no problem. Drove it about 10 miles this morning to work; no problem. Why does it appear to have corrected itself? Is the thermostat bad? Why does the temperature gauge not show that the car's overheating? Taking it to my mechanic this afternoon; just wanted some input. Thanks in advance.